In U.S. patent application Ser. No. 051,853 filed on May 19, 1987, now U.S. Pat. No. 5,093,415 a process is provided for producing stereoregular polymers. More particularly, it was found that polymers having an isotactic index of at least 96 percent could be prepared in high yield at high production rates by polymerizing an alpha-olefin in a low pressure gas phase fluidized bed process at temperatures in excess of 50.degree. C. employing a catalyst system comprising (i) a solid catalyst precursor, which includes magnesium, titanium, halogen, and an inside electron donor, i.e., a polycarboxylic acid ester containing two coplanar ester groups attached to adjacent carbon atoms; (ii) a hydrocarbylaluminum cocatalyst; and (iii) an outside electron donor or selectivity control agent, i.e., a silicon compound containing a silicon-oxygen-carbon linkage wherein the atomic ratio of aluminum to silicon is in the range of about 0.5:1 to about 100:1 and the atomic ratio of aluminum to titanium is in the range of about 5:1 to about 300:1.
This catalyst system can be used to provide homopolymers of propylene and copolymers of propylene and one or more alpha-olefins. The high activity at temperatures in the range of about 50.degree. C. to about 110.degree. C., which makes this catalyst system so attractive, is also responsible for a negative characteristic. This deficiency exhibits itself in the form of a kinetic profile, which can be described as "autoacceleration", and leads to processes, which are, in effect, out of control; processes in which the reactants are "over reactive"; agglomeration of particular product; and other various undesirable results such as hot spotting, chunking, and sheeting.
Autoacceleration can be defined as an abrupt increase in the reaction rate of a process to an undesirable level due to a sudden rise in temperature. In effect, the system is unable to remove the heat as fast as it is generated. Control of this behavior is essential for the smooth operation of the process in the reactor. Also, in the event of downstream process upsets, e.g., purge bin pluggage or extruder failure, it is desirable to quickly slow the polymerization rate to keep the reactor(s) or purge bin from overfilling with resin.
This can be accomplished by introducing a kill agent such as carbon monoxide into the polymerization reactor; however, carbon monoxide and similar kill agents require the purging of the reactor gases before the polymerization process can be restarted.
It would be advantageous to provide a kill agent, which not only avoids the problem of purging, but is at least partially reversible, and can also be used to minimize off-grade resin production. A reversible kill agent can be defined as a chemical which, when added to the polymerization reactor, results in a dramatic decrease in propagation rate (slow down in the rate of polymerization) without affecting the catalyst decay, with the stipulation that the polymerization can be restarted easily. A partially reversible kill agent is one in which there is an increase in the catalyst decay rate even though there is a decrease in monomer propagation rate. Again, the polymerization can be restarted without difficulty.